Propagators for gauge-invariant observables in cosmology

Understand

We make a proposal for gauge-invariant observables in perturbative quantum gravity in cosmological spacetimes, building on the recent work of Brunetti et al.

• [JHEP 08 (2016) 032].
• These observables are relational, and are obtained by evaluating the field operator in a field-dependent coordinate system.
• We show that it is possible to define this coordinate system such that the non-localities inherent in any higher-order observable in quantum gravity are causal, i.e., the value of the gauge-invariant observable at a point $x$ only depends on the metric and inflation perturbations in the past light cone of $x$.